James Cameron created Avatar as a fertile playground in which he can explore all of the ideas he wants to explore right now and get them on screen, and he has had resounding success in doing so – clearly audiences are eating these movies up.
I'd be hard pressed to imagine him throwing all of that away to return to a series that he didn't create and hasn't been involved with since 1986, and I'd be equally hard pressed to imagine that anyone that isn't into James Cameron's contemporary work would even be fond of what he'd likely do with a new Alien film nowadays anyways.